U.S. patent application number 10/467278 was filed with the patent office on 2004-03-11 for method for the online determination of hydrogen peroxide.
Invention is credited to Beuermann, Thomas, Teles, Joaquim Henrique.
Application Number | 20040048329 10/467278 |
Document ID | / |
Family ID | 7673159 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040048329 |
Kind Code |
A1 |
Beuermann, Thomas ; et
al. |
March 11, 2004 |
Method for the online determination of hydrogen peroxide
Abstract
A process for the on-line determination of the hydrogen peroxide
content of a mixture obtained in a chemical reaction comprises at
least the following steps: (1) admixing the mixture comprising
hydrogen peroxide with at least one reagent which is capable of
forming a substance which can be detected by optical methods on
reaction with hydrogen peroxide, so as to form this substance, (2)
determining the amount of the substance present by measuring its
specific absorption in an appropriate wavelength range.
Inventors: |
Beuermann, Thomas;
(Mannheim, DE) ; Teles, Joaquim Henrique;
(Otterstadt, DE) |
Correspondence
Address: |
Oblon Spivak McClelland Maier & Neustadt
Fourth Floor
1755 Jefferson Davis Highway
Arlington
VA
22202
US
|
Family ID: |
7673159 |
Appl. No.: |
10/467278 |
Filed: |
August 6, 2003 |
PCT Filed: |
February 5, 2002 |
PCT NO: |
PCT/EP02/01178 |
Current U.S.
Class: |
435/28 ;
424/600 |
Current CPC
Class: |
G01N 21/75 20130101;
Y10T 436/206664 20150115 |
Class at
Publication: |
435/028 ;
424/600 |
International
Class: |
C12Q 001/28; A61K
033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2001 |
DE |
10105528.5 |
Claims
We claim:
1. A process for the on-line determination of the hydrogen peroxide
content of a mixture obtained in a chemical reaction, which
comprises at least the following steps: (1) admixing the mixture
comprising hydrogen peroxide with at least one reagent which is
capable of forming a substance which can be detected by optical
methods on reaction with hydrogen peroxide, so as to form this
substance, (2) determining the amount of the substance present by
measuring its specific absorption in an appropriate wavelength
range.
2. A process as claimed in claim 1, wherein the at least one
reagent is selected from the group consisting of metals of
transition groups IV to IX of the Periodic Table of the
Elements.
3. A process as claimed in claim 2, wherein the at least one
reagent is selected from the group consisting of titanium-,
cobalt-, chromium-, zirconium-, hafnium-, vanadium-, niobium- and
tantalum-containing compounds.
4. A process as claimed in any of claims 1 to 3, wherein the at
least one reagent comprises a leuco dye and a peroxidase.
5. A process as claimed in any of claims 1 to 4, wherein the
determination of the amount of the substance present is carried out
by measuring its specific absorption and/or fluorescence in the
UV/VIS region of the spectrum.
6. A process for the oxidation of an alkene, which comprises
reacting the alkene with hydrogen peroxide in the presence of a
catalyst, wherein the hydrogen peroxide content of the reaction
mixture is determined on-line during the reaction by means of a
process as claimed in any of claims 1 to 5.
7. A process as claimed in claim 6, wherein the catalyst is a
zeolite catalyst.
8. A process as claimed in claim 6 or 7, wherein the zeolite
catalyst is a titanium silicalite having an MFI structure.
9. A process as claimed in any of claims 6 to 8, wherein propene is
reacted with hydrogen peroxide in methanolic solution in the
presence of a titanium silicalite having an MFI structure to form
propylene oxide.
10. A process as claimed in any of claims 6 to 9, wherein the
reaction is carried out in a plurality of reactors and the hydrogen
peroxide content is determined by means of a process as claimed in
any of claims 1 to 5 in at least one reactor, preferably all
reactors.
Description
[0001] The present invention relates to an on-line method of
determining the hydrogen peroxide content of a mixture obtained in
a chemical reaction and also to a process for the oxidation of an
alkene by means of hydrogen peroxide in the presence of a zeolite
catalyst into which this method is integrated.
[0002] The conversion of starting materials in chemical synthesis
is generally determined from their concentration in the synthesis
solution or the product mixture from the synthesis. Particularly in
the case of reactions carried out continuously, it is often
desirable to keep the conversion of one or more starting materials
within narrow limits. To achieve this, it is necessary to take
samples from the synthesis mixture at particular time intervals
during the synthesis process and to determine the concentration of
the respective starting material in these samples. If the residual
concentration of the starting material found in the product mixture
deviates from the optimum prescribed value, it is possible, for
example, to readjust the addition of the starting materials or to
modify the reaction conditions (e.g. temperature or pH) to bring
the conversion back to the desired value.
[0003] A particularly important case is oxidation reactions using
hydrogen peroxide.
[0004] To determine the concentration of hydrogen peroxide in a
mixture obtained in a chemical reaction, many analytical methods
are generally available to a person skilled in the art. In
principle, hydrogen peroxide can be determined both quantitatively
and qualitatively by taking advantage of its oxidizing and reducing
properties.
[0005] For example, hydrogen peroxide can be detected qualitatively
by means of the reaction with a titanium(IV) oxide sulfate
solution. An intensive yellow color develops as a result of the
formation of a titanium(IV) peroxy complex. Another qualitative
method of detection is the reaction with potassium dichromate
solution and dilute sulfuric acid, which results in the reaction
solution becoming blue.
[0006] On the other hand, quantitative determination is carried
out, for example, by means of oximetric titration with potassium
permanganate, sodium iodide or cerium(IV) sulfate, as is well known
to a person skilled in the art.
[0007] Difficulties in the analysis of hydrogen peroxide can arise
from the presence of organic hydroperoxides, for example
hydroperoxypropanols, in the analysis solution, since these also
react with KMnO.sub.4, NaI and cerium(IV) sulfate.
[0008] The determination of hydrogen peroxide is likewise
problematical when the amount to be determined is very low, i.e.
generally below 1% by weight.
[0009] A further problem occurs when the mixture in which the
concentration of hydrogen peroxide is to be determined is very
complex in nature, i.e. when further substances apart from the
substance to be determined are present in the analysis solution.
Examples which may be mentioned are further unreacted starting
materials and by-products formed in addition to the main
product.
[0010] Thus, a determination of hydrogen peroxide in synthesis
solutions having a low concentration of hydrogen peroxide or/and a
complex nature can in general not be carried out directly by means
of measurements such as optical absorption, reflection or emission,
for example fluorescence or phosphorescence.
[0011] Furthermore, measurement of the absorption of hydrogen
peroxide in the near infrared region, e.g. FT-NIR spectroscopy in
the range from 4631 to 5140 cm.sup.-1, fails for the above reasons
and also due to interference by water and organic
hydroperoxides.
[0012] Furthermore, measurement of the absorption of hydrogen
peroxide in the ultraviolet region, e.g. measurement of the UV
absorption at 254 or 280 nm, is suitable for the reliable
determination of the concentration of hydrogen peroxide only in
water/methanol solutions. If organic hydrogen peroxides are present
in the synthesis solution, reliable analysis of hydrogen peroxide
by this method is no longer possible.
[0013] In contrast, colorimetric methods are well suited to the
selective determination of hydrogen peroxide in synthesis solutions
having a complex nature and/or a low hydrogen peroxide content. In
these methods, the sample to be analyzed is reacted with a suitable
reagent to form a substance which absorbs or fluoresces in the
UV/VIS region of the spectrum and can thus be determined.
[0014] For example, the determination of hydrogen peroxide by the
"enzymatic method using peroxidase" is based on the oxidation of a
leuco dye (for example leuco-crystal violet) by the hydrogen
peroxide to be determined. This reaction is catalyzed by a
peroxidase (e.g. horseradish peroxidase type II, EC No. 1.11.1.7
(AS reg. No. 9003-99-0). The dye formed is subsequently determined
photometrically, e.g. at 596 nm in the case of leuco-crystal
violet. This method is specific for hydrogen peroxide. Only
hydroperoxides which liberate hydrogen peroxide extremely easily in
solution lead to interferences. This method is described in detail
in H. U. Bergmeyer, "Methoden der enzymatischen Analyse", 3rd
edition, Vol. II, p. 2297ff., Verlag Chemie, Weinheim (1974).
[0015] The "titanium sulfate method" is also suitable for the
quantitative colorimetric determination of hydrogen peroxide in the
presence of organic hydroperoxides. This determination is adversely
affected only by hydroperoxides which tend to liberate hydrogen
peroxide in an acidic medium (e.g. hemiperacetals). This method is
based on the yellow titanyl peroxo complex formed by the reaction
of hydrogen peroxide present with a titanium(IV) reagent (for
example titanyl sulfate, titanium(IV) chloride or potassium titanyl
oxalate). The titanyl peroxo complex has a strong absorption at
about 408 nm. Details of the way in which this method can be
carried out may be found in the literature, for example in
Kak{haeck over (c)}, Z. Vejdelek, "Handbuch der photometrischen
Analyse organischer Verbindungen", Volume 1, p. 92ff., Verlag
Chemie, Weinheim (1974) or in G. M. Eisenberg. Ind. Eng. Chem.,
Anal. Ed. (1943) 15, 327, and the literature references cited
therein.
[0016] This method is, as also described in the article by C. B.
Allsopp in "Analyst" (1941) 66, 371, suitable for determining
hydrogen peroxide in a very low concentration range down to
10.sup.-5 N.
[0017] A third calorimetric method which is likewise suitable for
the selective determination of hydrogen peroxide is the "cobalt
hydrogen carbonate method". This is based on the reaction of
hydrogen peroxide with Co(II) ions to form a colored cobalt peroxo
complex. This has a very strong absorption at about 260 nm. A
precise description of this method may be found in the literature
known to those skilled in the art, for example in Masschelen, W.
"Spectrophotometric Determination of Residual Hydrogen Peroxide",
Water and Sewerage Works, p. 69, August 1977.
[0018] The sensitivity of this method is very high, so that, for
example, hydrogen peroxide concentrations of about 0.02 ppm can be
detected, but certain compounds formed, for example, as by-products
of the oxidation of propylene by hydrogen peroxide (e.g. acetone,
which has a cutoff at a wavelength of .lambda..sub.c<330 nm)
also absorb in this region and thus interfere in the hydrogen
peroxide determination. This is a particular problem at very low
concentrations of hydrogen peroxide of about <100 ppm of
H.sub.2O.sub.2.
[0019] When carrying out such reactions, for example the oxidation
of an alkene by means of hydrogen peroxide in the presence of a
zeolite catalyst, an optimum yield should be achieved, but at the
same time reliable control of the reaction has to be ensured. To
achieve this, it is necessary to monitor the hydrogen peroxide
concentration on-line.
[0020] In general, an analytical method whose end points can be
indicated photometrically can also be carried out by means of
automated analytical instruments. Their use in a synthesis process
has hitherto frequently been separated from the data processing
unit for evaluation or process control and is coupled with this
only indirectly, i.e. "off-line", if at all.
[0021] The "off-line" analytical methods used hitherto for
determining hydrogen peroxide in synthesis processes during
operation have the disadvantage that there is a time delay between
the determination of the hydrogen peroxide concentration and the
necessary adjustment of the synthesis conditions.
[0022] Due to this time delay, the synthesis conditions cannot be
adapted to the changed hydrogen peroxide content sufficiently
quickly. Thus, the synthesis does not proceed optimally for this
time until the adjustment is made. During this time, increased
formation of by-products, for example, can occur. These in turn
reduce the yield of the actual product and make its work-up more
difficult.
[0023] For the reasons mentioned, it has hitherto been virtually
impossible to improve the economic efficiency of synthesis
processes, for example the oxidation of alkenes by means of
hydrogen peroxide in the presence of a catalyst with respect to the
yield and purity of the product, by monitoring the hydrogen
peroxide concentration.
[0024] Furthermore, especially in the case of starting materials
such as hydrogen peroxide which tend to undergo exothermic
decomposition reactions, safety considerations make it undesirable
to allow their concentration in the synthesis mixture to remain
uncontrolled for any length of time. Unreacted hydrogen peroxide
which is thus present in the output from the synthesis can lead to
unsafe situations such as explosive decomposition reactions, for
example in the subsequent work-up of the product.
[0025] It would therefore be advantageous, especially in the case
of batch or semibatch reactions, to have on-line monitoring of the
hydrogen peroxide concentration available so as to be able to avoid
accumulation of hydrogen peroxide to unacceptably high
concentrations which could represent a safety risk.
[0026] In the case of an oxidation in a continuous reactor, it is
important to monitor the residual content (and thus also the
conversion) of hydrogen peroxide in real time and to keep it within
predetermined limits by means of alterations to the reaction
conditions (e.g. temperature, pH or amounts of starting material).
This, too, could be achieved by means of an appropriate on-line
determination.
[0027] An on-line determination of the hydrogen peroxide content
would be of particularly great importance when the oxidation
reaction is carried out continuously in the presence of a catalyst
whose catalytic activity is not constant (e.g. because of
deactivation of the catalyst). To adhere to a prescribed value for
the residual hydrogen peroxide content in this case, it is
absolutely necessary for the reaction conditions to be continually
adjusted to compensate for the changing catalytic activity.
[0028] It is an object of the present invention to provide an
on-line method of determining the hydrogen peroxide content and a
process for the oxidation of alkenes by means of hydrogen peroxide
into which this method is integrated, so as to enable the
abovementioned disadvantages to be avoided.
[0029] We have found that this object is achieved by an on-line
method of determining the hydrogen peroxide content of a mixture
obtained in a chemical reaction, which method comprises at least
the following steps:
[0030] (1) admixing the mixture comprising hydrogen peroxide with
at least one reagent which is capable of forming a substance which
can be detected by optical methods on reaction with hydrogen
peroxide, so as to form this substance,
[0031] (2) determining the amount of the substance present by
measuring its specific absorption in an appropriate wavelength
range,
[0032] and by a process for the oxidation of alkenes by means of
hydrogen peroxide in the presence of a catalyst into which this
method is integrated.
[0033] For the purposes of the present invention, the term "on-line
determination of the hydrogen peroxide content" encompasses all
methods and apparatuses which are suitable for the determination of
hydrogen peroxide and which are directly connected to at least one
data processing unit so that it is possible to make adjustments to
a synthesis process in order to regulate it. A preferred apparatus
for the "on-line determination of the hydrogen peroxide content" is
described in DE 101 21 194.5 which was also filed by the applicant
of this invention.
[0034] The expression "directly" encompasses all ways known to a
person skilled in the art in which an instrument can be connected
to at least one data processing unit. This connection can, for
example, be via further equipment items known to a person skilled
in the art. These are able, for example, to receive, to amplify, to
transform or to otherwise modulate the signals produced by the
analytical apparatus. Furthermore, they can be connected both with
one another and with the analytical apparatus and with the data
processing unit or units via commercial cable connections, infrared
interfaces or similar means of transmitting signals.
[0035] In the method of the present invention, the apparatus for
the on-line determination of hydrogen peroxide comprises at least
one sampling device for taking a sample from the reaction mixture
formed in the course of the synthesis, at least one apparatus for
sample preparation and at least one instrument which is capable of
determining the specific absorption of the sample in a suitable
wavelength range. The apparatus in question comprises at least one
component which is connected to the equipment items described and
is capable of performing the control function for the individual
equipment items so as to coordinate their operation.
[0036] For process control of the synthesis plant, the data
determined by the apparatus for the on-line determination of
hydrogen peroxide are evaluated by means of at least one data
processing unit connected to this apparatus and converted into
control commands for process control of the synthesis plant. These
control commands can then be passed on to the process control
system of the plant, which is connected to the at least one data
processing unit.
[0037] A typical mixture used in the present method comprises at
least the products formed in the reaction under consideration here,
intermediates, by-products and starting materials, e.g. hydrogen
peroxide.
[0038] Chemical reactions which produce the mixture in which
hydrogen peroxide is to be determined with the aid of the method of
the present invention can be all chemical reactions known to those
skilled in the art in which hydrogen peroxide is, for example, used
as starting material or can be formed as by-product or
intermediate.
[0039] To be able to determine the hydrogen peroxide content by
means of the method of the present invention, the mixture obtained
is firstly admixed with at least one reagent which reacts with
hydrogen peroxide to form a substance which can be detected by
optical methods.
[0040] Reagents having this property are in principle all compounds
known to those skilled in the art for this purpose. They can be
used either individually or in admixture with one another or
together with further compounds. Further compounds can, for
example, comprise stabilizing or solubilizing additives.
[0041] In general, the formation of the substance involves reaction
of the reagent with hydrogen peroxide to form a complex or a
compound which can be detected by means of optical methods. Here,
the concentration of the substance and thus the hydrogen peroxide
content can be determined by comparison with a suitable
standard.
[0042] The reagent or reagents is/are preferably selected from the
group consisting of metals of transition groups IV to IX of the
Periodic Table of the Elements.
[0043] The reagent or reagents is/are more preferably selected from
the group consisting of titanium-, cobalt-, chromium-, zirconium-,
hafnium-, vanadium-, niobium- and tantalum-containing
compounds.
[0044] For example, the following reagents can be used for this
purpose: titanium(IV) compounds such as titanyl sulfate, cobalt(II)
salts, for example cobalt(II) sulfate or cobalt bicarbonate,
molybdenum(VI) salts, for example ammonium molybdate, vanadium
salts such as vanadyl sulfate, etc.
[0045] The reagent used is particularly preferably titanyl sulfate
or cobalt(II) sulfate.
[0046] In a further preferred embodiment of the present invention,
the reagent or reagents comprise(s) a leuco dye and a
peroxidase.
[0047] For the purposes of the present invention, the term "leuco
dye" refers to any dye whose oxidized form has a weaker or stronger
or other, e.g. shifted in terms of the absorption wavelength,
specific absorption in the optical spectrum compared to its reduced
form.
[0048] Preference is here given to using leuco-crystal violet
(tris(4-dimethylaminophenyl)-methane), leuco-malachite green
(bis(4-dimethylaminophenyl)phenylmethane), o-dianisidine and
acridinium salts, e.g. 10-methyl-.alpha.-(p-formylphenyl)acridinium
carboxylates and purpurogallin.
[0049] Particular preference is given to using leuco-crystal violet
(tris(4-dimethylaminophenyl)-methane) or purpurogallin.
[0050] In the reaction of hydrogen peroxide with the respective
leuco dye, the formation of a substance which is detectable by
optical methods proceeds via an electron transfer reaction which is
catalyzed by a peroxidase. The leuco dye is oxidized in this
reaction. The oxidized form of the leuco dye then absorbs at a
wavelength which is specific to it and can thus be detected by
means of optical methods. Here too, the concentration of oxidized
leuco dye and thus the concentration of hydrogen peroxide can be
determined by comparison with a suitable standard.
[0051] A peroxidase suitable for this embodiment of the present
invention is, for example, peroxidase type II (from horseradish),
which is commercially available.
[0052] Basically, the substance formed by reaction of the hydrogen
peroxide to be determined with the respective reagent can be
detected with the aid of appropriate optical methods on the basis
of its specific absorption in a particular wavelength range.
[0053] It is in principle possible to use all optical methods known
to those skilled in the art for the detection of a substance which
absorbs in a particular wavelength range.
[0054] Methods which may be mentioned by way of example are UV,
UV/VIS, VIS, IR, NIR and Raman spectroscopy. It is possible to
determine either the absorption (or transmission), the reflection
or the fluorescence.
[0055] In a preferred embodiment of the invention, the amount of
the substance present is determined by measuring its specific
absorption and/or fluorescence in the UV/VIS region of the
spectrum.
[0056] Instruments suitable for detection of the substance for the
purposes of the present invention are generally commercial
spectrometers appropriate for the respective wavelength range,
preferably spectrometers operating in the UV/VIS region.
[0057] The present invention further provides a process for the
oxidation of an alkene, which comprises reacting the alkene with
hydrogen peroxide in the presence of a catalyst, preferably a
zeolite catalyst. In this process, the hydrogen peroxide content of
the reaction mixture is determined on-line by means of the
above-described method during the reaction.
[0058] For the purposes of the present invention, the term "alkene"
refers to all compounds which have at least one C-C double
bond.
[0059] Examples of such organic compounds having at least one C-C
double bond are the following alkenes:
[0060] ethene, propene, 1-butene, 2-butene, isobutene, butadiene,
pentenes, piperylene, hexenes, hexadienes, heptenes, octenes,
diisobutene, trimethylpentene, nonenes, dodecene, tridecene,
tetradecene to eicosene, tripropene and tetrapropene,
polybutadienes, polyisobutenes, isoprenes, terpenes, geraniol,
linalool, linalyl acetate, methylenecyclopropane, cyclopentene,
cyclohexene, norbomene, cycloheptene, vinylcyclohexane,
vinyloxirane, vinylcyclohexene, styrene, cyclooctene,
cyclooctadiene, vinylnorbomene, indene, tetrahydroindene,
methylstyrene, dicyclopentadiene, divinylbenzene, cyclododecene,
cyclododecatriene, stilbene, diphenylbutadiene, Vitamin A,
beta-carotene, vinylidene fluoride, allyl halides, crotyl chloride,
methallyl chloride, dichlorobutene, allyl alcohol, methallyl
alcohol, butenols, butenediols, cyclopentenediols, pentenols,
octadienols, tridecenols, unsaturated steroids, ethoxyethene,
isoeugenols, anethole, unsaturated carboxylic acids such as acrylic
acid, methacrylic acid, crotonic acid, maleic acid, vinylacetic
acid, unsaturated fatty acids such as oleic acid, linoleic acid,
palmitic acid, naturally occurring fats and oils.
[0061] In the process of the invention, preference is given to
using alkenes containing from 2 to 18 carbon atoms. Particular
preference is given to reacting ethene, propene and butene. Very
particular preference is given to reacting propene.
[0062] The hydrogen peroxide used for the reaction with an alkene
in the process of the present invention can be prepared, for
example, with the aid of the anthraquinone process by means of
which virtually the entire amount of the hydrogen peroxide produced
worldwide is obtained. This process is based on the catalytic
hydrogenation of an anthraquinone compound to form the
corresponding anthrahydroquinone compound, subsequent reaction of
this with oxygen to form hydrogen peroxide and subsequent
extraction to separate off the hydrogen peroxide formed. The
catalysis cycle is closed by a new hydrogenation of the
anthraquinone compound which has been formed again in the reaction
with oxygen.
[0063] An overview of the anthraquinone process is given in
"Ullmanns Encyclopedia of Industrial Chemistry", 5th edition,
volume 13, pages 447 to 456.
[0064] It is likewise conceivable to obtain hydrogen peroxide by
conversion of sulfuric acid into peroxodisulfuric acid by anodic
oxidation with simultaneous evolution of hydrogen at the cathode.
Hydrolysis of the peroxodisulfuric acid then leads via
peroxosulfuric acid to hydrogen peroxide and sulfuric acid, which
is thus recovered.
[0065] Of course, preparation of hydrogen peroxide from the
elements is also possible.
[0066] Before use of hydrogen peroxide in the process of the
present invention, it is possible, for example, for a commercially
available hydrogen peroxide solution to be freed of undesirable
ions. Conceivable methods are, inter alia, methods as are
described, for example, in WO 98/54086, DE-A 42 22 109 or WO
92/06918. Likewise, at least one salt which is present in the
hydrogen peroxide solution can be removed from the hydrogen
peroxide solution by means of ion exchange in an apparatus
comprising at least one nonacidic ion exchange bed having a
cross-section of stream area A and a height H such that the height
H of the ion exchange bed is less than or equal to
2.5.times.A.sup.1/2, in particular less than or equal to
1.5.times.A.sup.1/2. For the purposes of the present invention, it
is in principle possible to use all nonacidic ion exchange beds
comprising cation exchangers or anion exchangers. It is also
possible to use mixed beds of cation and anion exchangers as ion
exchange beds. In a preferred embodiment of the present invention,
only one type of nonacidic ion exchangers is used. Further
preference is given to the use of a basic ion exchanger,
particularly preferably a basic anion exchanger and very
particularly preferably a weak base anion exchanger.
[0067] As regards the zeolite catalysts which can be used for the
purposes of the present invention, there are no particular
restrictions.
[0068] Zeolites are, as is known, crystalline aluminosilicates
having ordered channel and cage structures containing micropores
which are preferably smaller than about 0.9 nm. The network of such
zeolites is made up of SiO.sub.4 and AlO.sub.4 tetrahedra which are
joined via shared oxygen bridges. An overview of known structures
is given, for example, in W. M. Meier, D. H. Olson and Ch.
Baerlocher, "Atlas of Zeolite Structure Types", Elsevier, 4th
edition, London 1996.
[0069] Zeolites which contain no aluminum and in which the Si(IV)
in the silicate lattice is partly replaced by titanium as Ti(IV)
are also known. These titanium zeolites, in particular those having
a crystal structure of the MFI type, and possible ways of preparing
them are described, for example, in EP-A0 311 983 or EP-A 405 978.
Apart from silicon and titanium, such materials may further
comprise additional elements such as aluminum, zirconium, tin,
iron, cobalt, nickel, gallium, boron or small amounts of fluorine.
In the zeolite catalysts which are preferably regenerated using the
process of the present invention, the titanium of the zeolite can
be partly or completely replaced by vanadium, zirconium, chromium
or niobium or a mixture of two or more thereof. The molar ratio of
titanium and/or vanadium, zirconium, chromium or niobium to the sum
of silicon and titanium and/or vanadium and/or zirconium and/or
chromium and/or niobium is generally in the range from 0.01:1 to
0.1:1.
[0070] Titanium zeolites, especially those having a crystal
structure of the MFI type, and possible ways of preparing them are
described, for example, in WO 98/55228, WO 98/03394, WO 98/03395,
EP-A 0 311 983 or EP-A 0 405 978, whose relevant disclosure is
fully incorporated by reference into the present patent
application.
[0071] Titanium zeolites having an MFI structure can, as is known,
be identified by means of a particular X-ray diffraction pattern
and also by means of a lattice vibration band in the infrared
region (IR) at about 960 cm.sup.-1 and can thus be distinguished
from alkali metal titanates or crystalline and amorphous TiO.sub.2
phases.
[0072] Specific examples of suitable zeolites are titanium-,
germanium-, tellurium-, vanadium-, chromium-, niobium- and
zirconium-containing zeolites having a pentasil zeolite structure,
in particular the types which can be assigned
X-ray-crystallographically to the ABW, ACO, AEI, AEL, AEN, AET,
AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD,
AST, ATN, ATO, ATS, ATT, ATV, AWO, AWW, BEA, BIK, BOG, BPH, BRE,
CAN, CAS, CFI, CGF, CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO,
DFT, DOH, DON, EAB, EDI, EMT, EPI, ERI, ESV, EUO, FAU, FER, GIS,
GME, GOO, HEU, IFR, ISV, ITE, JBW, KFI, LAU, LEV, LIO, LOS, LOV,
LTA, LTL, LTN, MAZ, MEI, MEL, MEP, MER, MFI, MFS, MON, MOR, MSO,
MTF, MTN, MTT, MTW, MWW, NAT, NES, NON, OFF, OSI, PAR, PAU, PHI,
RHO, RON, RSN, RTE, RTH, RUT, SAO, SAT, SBE, SBS, SBT, SFF, SGT,
SOD, STF, STI, STT, TER, THO, TON, TSC, VET, VFI, VNI, VSV, WIE,
WEN, YUG, ZON structures and to mixed structures made up of two or
more of the abovementioned structures. It is also possible for
titanium-containing zeolites having the ITQ-4, SSZ-24, TIM-1,
UTD-1, CIT-1 or CIT-5 structure to be used in the process of the
present invention. Further titanium-containing zeolites which may
be mentioned are those having the ZSM-48 or ZSM-12 structures.
[0073] For the purposes of the present invention, preference is
given to using Ti-zeolites having an MFI, MEL or mixed MFI/MEL
structure. Preference is also given specifically to the
Ti-containing zeolite catalysts generally referred to as "TS-1",
"TS-2", "TS-3", and also Ti zeolites having a framework structure
which is isomorphous with .beta.-zeolite.
[0074] Accordingly, the present invention also provides a process
as described above in which the catalyst is a titanium silicalite
having the TS-1 structure.
[0075] When carrying out the process, it has surprisingly been
found that the on-line determination of the hydrogen peroxide
content enables the running synthesis process to be optimally
controlled. The determination is, in particular, carried out on the
product mixture from the reaction. In a synthesis carried out in a
number of reactors connected in series, the determination is
preferably carried out on the product mixture from either
individual, selected reactors or from all reactors. In this way,
the total yield or the yield from each reactor and the purity of
the respective reaction mixture and thus also of the product can be
optimized. Furthermore, the safety risk resulting from any hydrogen
peroxide present in the product mixture from a synthesis without
optimized control can be minimized.
[0076] For the purposes of the present invention, the determination
of the hydrogen peroxide content during the course of the process
of the present invention is carried out mainly periodically,
preferably at a frequency in the range from 0.5 to 100 h.sup.-1.
Particular preference is given to frequencies in the range from 1
to 60 h.sup.-1.
[0077] For example, on the basis of the relationship between the
following factors:
[0078] the temperature prevailing in the reactor,
[0079] the conversion of hydrogen peroxide and
[0080] the catalytic activity of the catalyst used in the
reaction,
[0081] deactivation of the catalyst used in each case can be
countered by continuously increasing the temperature or adjusting
the pH, as described in DE 199 36 547.4.
[0082] The temperature increase required for each regulated step is
determined with the aid of the determination of the hydrogen
peroxide content of the respective reactor output carried out at
short time intervals. As a result of the on-line connection between
the apparatus for determining the concentration of hydrogen
peroxide and the process control system of the synthesis plant, the
necessary regulation occurs without any significant time delay.
[0083] As a consequence, the conversion of hydrogen peroxide can be
kept constant over virtually the entire synthesis process. This in
turn has a positive effect on the yield and purity of the desired
product.
[0084] In the case of a plurality of reactors connected in series,
it is of course also possible, in the context of the invention, to
determine the hydrogen peroxide content in each reactor or in the
output from each reactor by means of the appropriate number of
apparatuses for the on-line determination of the hydrogen peroxide
content. One or more data processing units connected to these
apparatuses transmit(s) control commands corresponding to the
hydrogen peroxide content determined in each case to the process
control for the respective reactor. The process control system can
thus make regulating adjustments when required.
[0085] Accordingly, the present invention also provides a process
for the oxidation of an alkene of the type according to the
invention in which the reaction is carried out in a plurality of
reactors. In this process, the concentration of hydrogen peroxide
in at least one reactor, preferably all reactors, can be determined
by means of the novel on-line method of determining the hydrogen
peroxide content.
[0086] The process of the present invention, as described above, is
preferably used for the conversion of propene into propylene oxide
by means of hydrogen peroxide in methanolic solution in the
presence of a titanium silicalite having an MFI structure.
[0087] In a particularly preferred embodiment of this reaction to
form propylene oxide, propylene is reacted with hydrogen peroxide
in the presence of methanol, a basic salt and TS-1 as catalyst in a
first reactor (main reactor, for example tubular reactor).
[0088] Here, the reaction pressure is selected and kept constant at
a value at which no gas phase is present during the reaction. The
temperature is selected so that the hydrogen peroxide conversion in
the output from the reactor is from 85 to 95%, preferably from 88
to 93%.
[0089] Since the catalyst is typically deactivated during the
course of the reaction, the temperature has to be continuously
adjusted for the abovementioned reasons. In general, the
temperature increase necessary is from 0.2 to 1.5.degree. C. per
day, depending on the reaction conditions.
[0090] To determine the precise temperature increase necessary at a
given point in time during the synthesis, the hydrogen peroxide
conversion is determined at short time intervals, as described
above.
[0091] For example, the output from the first reactor is worked up
in a distillation column in which at least 90%, typically >99%,
of the propylene oxide formed are separated off at the top.
[0092] The remaining bottoms are admixed with propylene and, if
appropriate, with a basic salt and reacted in a second reactor
(after-reactor, for example a simple tube reactor or shaft
reactor). In the after-reactor, preferably from 90 to 95% of the
hydrogen peroxide introduced into it are reacted, since lower
conversions frequently leave residual hydrogen peroxide which can
cause safety problems, while higher conversions frequently result
in a decrease in the selectivity of the reaction.
[0093] To control the conversion in the after-reactor, adjustments
are made, for example, to the inlet temperature or the amount of
base via the process control system in accordance with the hydrogen
peroxide content determined on-line.
[0094] The invention is illustrated by the examples below.
EXAMPLE 1
[0095] Apparatus for the On-Line Determination of Hydrogen
Peroxide
[0096] The overall apparatus for the on-line UVNIS-spectroscopic
determination of H.sub.2O.sub.2 comprises:
[0097] 1. a metering and control system (e.g. process titrator from
the "ADI" series from Metrohm) for automatic sampling and carrying
out the color reaction,
[0098] 2. a fiber optic transmission probe which dips into the
reaction vessel and is connected via silica optic fibers to
[0099] 3. a UVVIS spectrometer (preferably a diode array
spectrometer) for recording the spectra, and also
[0100] 4. a computer (PC) for evaluating the spectra and
calculating the H.sub.2O.sub.2 concentration.
[0101] 5. The H.sub.2O.sub.2 concentrations in the product stream
which have been determined in this way can subsequently be
converted by means of a digital/analog converter into a 4-20 mA
electric signal which is transmitted to a process control system
for controlling the plant.
[0102] The schematic structure is shown in FIG. 1.
[0103] The UV/VIS spectrometer is preferably triggered by the
process titrator. In this case, a transmission probe dips into each
of the reaction vessels in the process titrators. If these
transmission probes are connected via optic fibers (preferably made
of quartz) to an optical multichannel multiplexer, one spectrometer
is generally sufficient for (virtually) simultaneously recording
the absorption spectra at the various measurement points.
[0104] Procedure for the H.sub.2O.sub.2 Determination:
[0105] A few milliliters of sample (typically 0.5-5 ml, depending
on the concentration) are taken from the product stream via a
capillary line with the aid of the metering system and are
transferred into the reaction vessel located in the titrator. The
color reagent (commercially available titanyl sulfate solution,
about 5% by weight of Ti) (typically 0.5-5 ml) is added from a
reservoir. A yellowish titanyl peroxo complex is formed after a
short time (typically 1 min). The solution is subsequently made up
to a specific volume (typically in the range from 25 to 500 ml)
with a solvent (e.g. distilled water, dilute sulfuric acid,
etc.).
[0106] To record the UV/VIS absorption spectrum, it is necessary
for the measuring slit of the transmission probe to dip completely
into the solution to be analyzed. A typical UV spectrum is shown in
FIG. 2. A spectrum previously recorded on the solvent serves as
reference (=100% transmission).
[0107] The conversion of the measured UV/VIS absorbances into
H.sub.2O.sub.2 concentrations is carried out on the computer by
means of a measurement and evaluation program with the aid of a
calibration function. The calculation is preferably carried out
using the absorbance at the absorption maximum of the titanyl
peroxo complex at about 408 nm.
EXAMPLE 2
[0108] Sequence of Steps for the Photometric On-Line Determination
of H.sub.2O.sub.2
[0109] For the on-line determination of hydrogen peroxide in the
output from a reactor for the epoxidation of propylene, an
apparatus as described in example 1, FIG. 1 was constructed. This
comprised the following components:
[0110] metering system (ADI 2015 from Metrohm) with control unit
and measurement cell,
[0111] U/VIS diode array spectrometer (MCS521 from Zeiss),
[0112] computer (spectrometer control, evaluation, data transfer to
PCS),
[0113] optical immersion probe (from Hellma).
[0114] The actual measurement was carried out according to the
sequence described below:
[0115] 1. emptying of the measurement cell (reaction vessel),
[0116] 2. filling of the measurement cell with solvent (e.g. water)
("blank"),
[0117] 3. trigger signal to UV/VIS process spectrometer for
recording the
[0118] 4. reference spectrum by means of the transmission
probe,
[0119] 5. measurement cell emptied by means of suction,
[0120] 6. sample (typically 0.5-5 ml, depending on H.sub.2O.sub.2
concentration) transferred (sample drawn by suction via a capillary
line from the sampling point and metered into the measurement
cell),
[0121] 7. reagent, viz. titanyl sulfate solution containing about
5% by weight of Ti (typically 0.5-5 ml), depending on
H.sub.2O.sub.2 concentration, is metered into the measurement
cell,
[0122] 8. pause (a few seconds at room temperature) to allow
formation of the titanyl peroxo complex,
[0123] 9. solution made up to a specific volume (typically 25-500
ml) with solvent (e.g. water),
[0124] 10. pause,
[0125] 11. trigger signal to spectrometer for recording the UV/VIS
absorption spectrum of the reaction solution,
[0126] 12. evaluation program on the computer calculates the
H.sub.2O.sub.2 concentration from the absorbance,
[0127] 13. transmission of the H.sub.2O.sub.2 concentration from
the computer to the process control system of the synthesis
plant,
[0128] 14. emptying of the measurement cell,
[0129] 15. rinsing of the measurement cell with solvent.
[0130] After a defined delay time, the sequence starts afresh at
item 1.
EXAMPLE 3
[0131] The epoxidation of propylene by means of hydrogen peroxide
was carried out in a tube reactor which had a diameter of 45 mm and
a length of 2 m, was provided with a cooling jacket and was charged
with about 620 g of a fresh epoxidation catalyst (titanium
silicalite TS-1 in the form of extrudates having a diameter of 1.5
mm). The amounts of the individual starting materials used were as
follows:
1 Methanol: 1834 g/h Hydrogen peroxide (40% in water): 332 g/h
Propene: 244 g/h K.sub.2HPO.sub.4 solution (1.25% by weight in
water): 4 g/h
[0132] The individual starting materials were combined under
superatmospheric pressure (about 20 bar) upstream of the reactor
and then passed through the reactor. The temperature of the cooling
medium in the jacket was about 30.degree. C. at the beginning of
the experiment. When the conversion began to decrease, the
temperature of the cooling medium was adjusted so that a constant
conversion of hydrogen peroxide was achieved. The conversion was
determined by on-line determination of the hydrogen peroxide in the
output from the reactor, as described in examples 1 and 2.
[0133] An additional sample was taken once per day and analyzed
off-line by the titanyl sulfate method to provide a comparison. The
comparison of the results of the on-line and off-line
determinations during the experiment over virtually 800 hours is
shown in FIG. 3.
[0134] FIG. 3 shows a comparison of the hydrogen peroxide content
of the output from the reactor described in the example determined
by the on-line and off-line methods. (The on-line method is
indicated in FIG. 3 by means of a line, while the off-line method
is indicated by circular measurement points.)
[0135] The good agreement between the on-line and off-line
determinations of the hydrogen peroxide content demonstrates the
quality of the measurement. A purely off-line determination would
not be sufficient to achieve the desired continuous adjustment of
the temperature or to keep the hydrogen peroxide conversion
constant.
[0136] Reference Numerals for FIG. 1
[0137] 1=Line for taking the sample
[0138] 2=Sampling valve
[0139] 3=Measurement cell
[0140] 4=Transmission probe
[0141] 5=Optic fibers
[0142] 6=Trigger signal
[0143] 7=Spectrometer
[0144] 8=Light source
[0145] 9=Signal proportional to the H.sub.2O.sub.2 concentration
from computer or data processing system to process control
system
[0146] 10=Control commands from process control system to the
process
[0147] 11=Propylene oxide plant
[0148] 12=Process titrator
[0149] 13=Discharge to waste
[0150] 14=Solvent
[0151] 15=Color reagent
* * * * *